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SubjectRe: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates
----- On Jul 28, 2021, at 4:28 PM, paulmck paulmck@kernel.org wrote:

> On Wed, Jul 28, 2021 at 04:03:02PM -0400, Mathieu Desnoyers wrote:
>> ----- On Jul 28, 2021, at 3:45 PM, paulmck paulmck@kernel.org wrote:
>> [...]
>> >
>> > And how about like this?
>> >
>> > Thanx, Paul
>> >
>> > ------------------------------------------------------------------------
>> >
>> > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3
>> > Author: Paul E. McKenney <paulmck@kernel.org>
>> > Date: Wed Jul 28 12:38:42 2021 -0700
>> >
>> > rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
>> >
>> > The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
>> > counter of an incoming CPU if required. It is currently is invoked
>>
>> "is currently is" -> "is currently"
>
> Good catch, fixed!
>
>> > from rcutree_prepare_cpu(), which runs before the incoming CPU is
>> > running, and thus on some other CPU. This makes the per-CPU accesses in
>> > rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
>> > the running CPU cannot possibly be in dyntick-idle mode, which means
>> > that rcu_dynticks_eqs_online() never has any effect. One could argue
>> > that this means that rcu_dynticks_eqs_online() is unnecessary, however,
>> > removing it makes the CPU-online process vulnerable to slight changes
>> > in the CPU-offline process.
>>
>> Why favor moving this from the prepare_cpu to the cpu_starting hotplug step,
>> rather than using the target cpu's rdp from rcutree_prepare_cpu ? Maybe there
>> was a good reason for having this very early in the prepare_cpu step ?
>
> Some years back, there was a good reason. This reason was that
> rcutree_prepare_cpu() marked the CPU as being online from an RCU
> viewpoint. But now rcu_cpu_starting() is the one that marks the CPU as
> being online, so the ->dynticks check can be deferred to this function.
>
>> Also, the commit message refers to this bug as having no effect because the
>> running CPU cannot possibly be in dyntick-idle mode. I understand that calling
>> this function was indeed effect-less, but then why is it OK for the CPU coming
>> online to skip this call in the first place ? This commit message hints at
>> "slight changes in the CPU-offline process" which could break it, but therer is
>> no explanation of what makes this not an actual bug fix.
>
> Because rcutorture would not have suffered in silence had this
> situation ever arisen.

Testing can usually prove the presence of a bug, but it's rather tricky to prove
the absence of bug.

>
> I have updated the commit log to answer these questions as shown
> below. Thoughts?

I'm still concerned about one scenario wrt moving rcu_dynticks_eqs_online()
from rcutree_prepare_cpu to rcu_cpu_starting. What happens if an interrupt
handler, or a NMI handler, nests early over the CPU-online startup code ?
AFAIU, this interrupt handler could contain RCU read-side critical sections,
but if the eqs state does not show the CPU as "online", I wonder whether it
will work as expected.

Thanks,

Mathieu

>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 516c8c4cc6fce62539f7e0182739812db4591c1d
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date: Wed Jul 28 12:38:42 2021 -0700
>
> rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
>
> The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
> counter of an incoming CPU when required. It is currently invoked
> from rcutree_prepare_cpu(), which runs before the incoming CPU is
> running, and thus on some other CPU. This makes the per-CPU accesses in
> rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
> the running CPU cannot possibly be in dyntick-idle mode, which means
> that rcu_dynticks_eqs_online() never has any effect.
>
> It is currently OK for rcu_dynticks_eqs_online() to have no effect, but
> only because the CPU-offline process just happens to leave ->dynticks in
> the correct state. After all, if ->dynticks were in the wrong state on a
> just-onlined CPU, rcutorture would complain bitterly the next time that
> CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y,
> for example, those built by rcutorture scenario TREE04. One could
> argue that this means that rcu_dynticks_eqs_online() is unnecessary,
> however, removing it would make the CPU-online process vulnerable to
> slight changes in the CPU-offline process.
>
> One could also ask why it is safe to move the rcu_dynticks_eqs_online()
> call so late in the CPU-online process. Indeed, there was a time when it
> would not have been safe, which does much to explain its current location.
> However, the marking of a CPU as online from an RCU perspective has long
> since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all
> that is required is that ->dynticks be set correctly by the time that
> the CPU is marked as online from an RCU perspective. After all, the RCU
> grace-period kthread does not check to see if offline CPUs are also idle.
> (In case you were curious, this is one reason why there is quiescent-state
> reporting as part of the offlining process.)
>
> This commit therefore moves the call to rcu_dynticks_eqs_online() from
> rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed
> to be running on the incoming CPU. The call to this function must of
> course be placed before this rcu_cpu_starting() announces this CPU's
> presence to RCU.
>
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 0172a5fd6d8de..aa00babdaf544 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4129,7 +4129,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
> rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
> rdp->blimit = blimit;
> rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */
> - rcu_dynticks_eqs_online();
> raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
>
> /*
> @@ -4249,6 +4248,7 @@ void rcu_cpu_starting(unsigned int cpu)
> mask = rdp->grpmask;
> WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
> + rcu_dynticks_eqs_online();
> smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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